Bob,
I agree, but I didn't really want to chime in until I was sure I hadn't missed something in your particular build. Also, I really think mixing the concepts of 'thermal mass' and 'insulation layer' just muddies the waters. The vermiculite layer is for insulation. The more portland you add, the, slightly, less insulating properties it has, perhaps. As far as adding thermal mass, if you really wanted to do that, then set your oven bricks on the 2.5" side and use 4.5" thick oven floor. Or build up a layer of 'fireclay mortar' one inch thick under the entire oven floor brick layer.

Personally, it's hard to know exactly how this will all perform. I ended up with a very thin layer of vermiculite under my floor. (2.5") I am crossing my fingers that the oven will still heat up and make pizza's well enough. I have a feeling I may have to throw in an extra log here and there, but, oh well. Without several firings, it will be hard to speak with any authority on the matter.

Well...Did our vermiculite/portland hearth tonight. It came out rather well. Because we did it in a wheel barrel we took a 1 gallon pail of cement to 5 of verm. to 1. 5 gallons of water. Mixing with a garden rake well first and then addin gthe water gave us a very good consistancy. We then shoveled it into the form and skreet it the best one can with the material that was like moist oatmeal. We took our flat steel trowel and pat it down. I can see how it gets its insulating quality. It has nice feel to it. We plan on starting the floor at the ens of the week if not then Sun is the day.
Bob, where have you found the fire clay and mesh sand? I assume mesh sand is just fine sand. Does anyone know why we couldn't use a slurry of refratory and sand in place of the fire clay. Any thoughts or substitutions?
G

G.
You can buy extremely fine sand at most Lowe's type stores. The fireclay is available at most pottery and clay working places. I ended up using refractory mortar, but I would have used fireclay had I known better ( the particular mortar I used was not that great -- but I am sure it will be fine... essentially, it's just to level the bricks) by the time you build an igloo on top of them, they will likely stay in place without any mortar. Isn't that vermiculite mixture cool?

Lars,
I will probably end up using the refractory cement and possibly adding a little sand to it. It's amazing that there are so few pottery/ceramic supply houses in our area. I too think that the primary purpose is to act as a leveling agent. I will end up with a little more than a 3" inch insulation hearth. I think that will be just fine. there is a couple around the block that built a pizza oven without refering to any plans and not having any knowledge that is afforded now and don't even think that they have an insulation hearth and they are using their oven effectively. I don't think that they have the high extended temps that we should achieve with our ovens but the bottom line is they are having a ball making pizzas for themselves, family and friends on a regular basis.
Will start the floor and dome this weekend, that's where the fun and swearing will start. Looking forward to it.Thanks for the info.
G

G.
In Connecticuit? Wow, I would think there would be MANY people doing clay, and several sources for fireclay. My first bag was hard to come by ( someone actually drove it up from KS ( 3 hrs.)) but later I found a quick source, and later yet another.

If you plan to mix your own mortar you will need about 90 lbs. I thought that I could not find it here when I first started looking, but I was surprised to find those several other places, and even a less expensive source for firebrick, ceramic blanket, etc. months after I first started looking.

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